Nanocrystal-containing filtration media

ABSTRACT

The invention relates to filtration media having nanocrystals of metal oxides, such as zinc oxide or titanium oxide or mixtures thereof, encapsulated in or impregnated into a binder matrix.

This application is a continuation-in-part of U.S. Ser. No. 09/805,758,filed Mar. 13, 2001, a continuation-in-part of U.S. Ser. No. 09/772,542,filed Jan. 30, 2001, and a continuation-in-part of U.S. Ser. No.09/560,824, filed Apr. 28, 2000, now abandoned, which is acontinuation-in-part of U.S. Ser. No. 08/819,999, filed Mar. 18, 1997,now U.S. Pat. No. 6,241,893, the entire contents of each of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a filtration media containing nanocrystals madeof metal oxide dispersed in a binder matrix for the removal ofmicroorganisms and other contaminants from water.

2. Description of Related Art

Drinking water, in some locations world-wide, contains bacteria andviruses that are harmful to humans, in many cases, rendering the waterunfit for consumption. There are a variety of different techniques andequipment that can reduce bacteria and viruses to certain acceptableperformance levels, such as ceramic filters, sub-micron filters, ionexchange resins, ultra-violet lights, ozonators, distillation equipment,and other apparatus. Microfiltration generally presents significantdrawbacks because of the large pressure drops involved and because ofthe limited capacity of the microfilters. With bacteria having sizes ofaround 0.1 micron, such as B. Diminuta, the performance of microfiltersis generally very poor, and clogging takes place in a short time.Consumers who use these filters to reduce bacteria generally must relyon increased pressure drop as the only indication that it is time toreplace the microfilter. There is no reliable method to determinewhether the filter will last 10, 50, 100 or 1000 gallons, or what theremaining capacity of a filter in use might be. Turbidity and thepresence of other contaminants than microorganisms can affect thesurface of the microfilter, which creates some limitations on the use ofthe filter. Ultra-violet lights are affected by scale buildup on thelamp sites and frequency changes that may effect their performance inbacteria reduction, and UV wavelength sensors are very expensive.

Filtration media are often assigned a “rating” based upon the size ofparticulates that can be removed from water using these filters. Typicaltesting to establish these ratings include NSF Class 1 Particulate andNSF 53 AC Dust testing. Reducing the ratings (desirable, because itindicates that smaller particles can be produced) generally requires theuse of specialized particles having very small pore sizes. Theseparticles become difficult and expensive to produce, so that decreasingthe nominal rating of the filtration media is limited by the expense ofthe particles necessary to include in the media. In addition, filtersthat have submicron ratings, and which function by occlusion, have veryshort lifetimes. For example, a 0.2 micron rated filter of approximately3 in. diameter and 10 in. length filtering New York City water at 1 gpmwill suffer reduced capacity and significantly increased pressure dropafter filtering only 100 gallons of water.

Recent advances in “hybrid” materials, i.e., nanostructured materialsthat contain both organic and inorganic components or moieties, has ledto the development of filtration materials capable of achievingsubmicron level removal of particulates as well as removal ofmicroorganisms, but that are capable of operating at high flow rates andfor extended periods of time without substantial degradation ofperformance. The invention described herein is one such material.

SUMMARY OF THE INVENTION

It has been found that combining nanocrystals of metal oxides, such aszinc oxide or titanium oxide or mixtures thereof, encapsulated in orimpregnated into a binder matrix. The binder matrix may be a polymericmaterial, and the metal oxide nanocrystals may be optionally mixed withcarbon and/or other organic particulates. The inclusion of the metaloxide nanoparticles significantly decreased the micron rating of thefiltration material as compared to the same material without thenanoparticles, and provided a material that is capable of reducinglevels of microorganisms, such as bacteria, including those having anaverage particle size ranging from about 0.1 to about 1 micron, at anefficiency of 99.999%.

Without wishing to be bound by any theory, it is believed that thefiltration media functions to remove microorganisms without significantsize exclusion of the microorganisms. Regardless of the exact mechanismby which the material functions, it allows the preparation of afiltration media that is capable of removing submicron contaminants atextremely high efficiency. The inclusion of metal oxide nanoparticles inthe filtration media allows the use of binder and, e.g., carbonparticulates suitable for achieving a micron or larger nominal rating,but in fact achieving submicron performance without diminished lifetime.By contrast with the 0.2 micron filter described above, a similar filterincluding metal oxide nanoparticles can process over 1000 gallons of thesame water at the same flow rate with less than a 30% pressure drop atthe end of processing.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

According to the invention, nanocrystals of metal oxides havingparticles sizes ranging from approximately 20 nm to 400 nm areincorporated into a filtration media containing a binder matrix.Desirably, this filtration media also contains some form of additionalparticulate, such as activated carbon, and the binder is desirably apolymeric material. The resulting filtration media is capable ofdestroying bacteria and other organisms having sizes below 1 micron.

The filtration media of the invention can be viewed as a microcoating ofmetal oxide nanocrystals on the surface of a polymeric binder (includinginternal surfaces, such as those provided by pores within the bindermatrix). The metal oxide nanocrystals are included in amounts rangingfrom approximately 0.1% up to about 10% by weight, based upon the weightof the entire filtration media. Suitable metal oxides include, but arenot limited to, zinc oxide, copper oxide, and titanium dioxide. Othermetal oxide nanocrystals may also be suitable, and this can bedetermined by preparing suitable filter blocks containing these metaloxide nanocrystals, as described herein, and testing the blocks againstsubmicron particles and against microorganisms, as described herein, todetermine their suitability.

The nanocrystals are believed to interact with the binder, which istypically a polymeric binder, such as high density polyethylene or lowdensity polyethylene or a mixture thereof. The nanocrystals aretypically combined with the polymer and, before or after the addition ofother optional components such as activated carbon, heated to atemperature ranging from about 150° C. to about 250° C. The nanocrystalscan be incorporated into the polymer by, e.g., high-speed shear mixingfor approximately 10-30 minutes in the mixer. The nanocrystals and thepolymeric binder in particulate form are simply added to the mixer inthe requisite quantities, and mixed. Activated carbon and optionallyzirconia can then be added. The order of addition is not critical,however it is generally desirable to add the nanocrystals to the binderprior to adding other components and prior to heating, in order toassure complete mixing. The resulting mixture is then heated to raisethe temperature of the polymeric binder. In general, the polymericbinder material containing the metal oxide nanocrystals is heated slowlyto form the filtration media. Polymeric binders containing the metaloxide nanocrystals are then heated from about 30 minutes to about 6hours at an approximate temperature of 550° F. in order to form a blockof filtration media.

PREPARATION EXAMPLE

High-density polyethylene particles (20 wt %, based on the totalmixture) are combined with 2 wt % ZnO nanocrystals having particle sizesin the range of about 20 nm to about 100 nm, and prepared by plasmavaporization, quenching, and cooling (i.e., by a “fuming” process), with7 wt % particulate zirconia, and the balance particulate active carbon.The mixture is filled into a round cylindrical metal alloy mold designedto evenly distribute heat across its surface. The material is thencompressed 100 psi under heating at a temperature of 500° F. for atleast about 1 hour, up to a maximum of about 6 hours, and then cooled toambient temperature. The resulting block of material can then be used asa filtration media that will reduce the level of microorganisms in afluid.

It has been found that quantities of titanium dioxide nanocrystals aslow as 7.5 micrograms demonstrated the capacity to repeatedly (over 56times) destroy E-coli colonies in water (30,000 counts in 1000 cc ofwater). An average sized filter prepared from the filtration media ofthe invention contains around 0.5% by weight, based on the total weightof the filtration media, of nanocrystals of zirconia or titania or both,and is capable of destroying tens of billions of bacteria. Thepercentage of metal oxide nanocrystals impregnated into the polymer candetermine the capacity of the filter for the reduction of bacteria andother microorganisms. For instance, passing approximately 400 gallons ofwater containing B. Diminuta at a concentration of 10,000 ct/cc at aflow rate of 0.5 to 1 gpm through a 2.5 in. diameter×10 in. cylindricalfilter with a nominal micron rating of approximately 1-5 micron andhaving around 0.5 wt % nanocrystals of titania or zinc oxide or mixturesthereof (based on the total weight of the filtration media) can reducethe bacteria count by 99.999%. This same filter can achieve the sameefficiency with water containing 1,000 ct/cc over the course of treatingapproximately 1,500 gallons of water at similar flow rates. Since theaverage toxic bacteria count in water under the worst expectedconditions would typically not exceed 1,000-3,000 ct/cc in drinkingwater (which is considered unacceptable for human consumption), thenanocrystals combined with zirconia can be calculated accurately for thecapacity of the filter of a given size with a given flow rate.

In addition, it has been found that incorporation of the nanocrystalsimproves the performance of the polymer-carbon filtration mediatremendously, possibly by controlling the complexing of the binder,and/or improving the surface structure of the carbon, polymeric binder,and metal oxide combination. Testing was conducted using a filtrationmedia produced by compressing coarse carbon having an average particlesize of approximately 50-100 microns, polymer binder particles having anaverage particle size of approximately 30 microns, with and without 0.2%of nanocrystalline titanium oxide. When the nanocrystalline titaniumoxide was included, a 99.999% efficiency one micron filter was obtained.Without the nanocrystalline titanium oxide, the rating of the filter wasapproximately 5-20 microns, more particularly 10 micron. The inclusionof 0.5% of nanocrystalline titanium oxide results in the ability to forma one-micron filter (i.e., a filter capable of removing 1 micronparticle dust in test water at a level of 50,000 ct/cc with anefficiency of 99.99%).

The 0.1 wt % nanocrystalline titanium oxide filter described above wastested by NSF for bacteria reduction.

Example 1

5000 gallons of water seeded with 30,000 ct/cc E. Coli were passedthrough a 169 cu. in. filter having a micron rating of 2, at 3gpm, withan inlet pressure of 60 psi and an outlet pressure of 52 psi. The filtercompositions were 0.5% nanocrystalline titanium oxide, 6% zirconiacrystals, 20% high density HDPE and the balance activated carbon. Thefiltration media resulted in a bacterial reduction efficiency of99.9999%.

Example 2

The test described above in Example 1 was also conducted by seeding thewater with 0.1 micron bacteria (B. diminuta) at a concentration ofapproximately 70,000 ct/cc, and using 4,000 gallons of water. Thefiltration media demonstrated a bacterial reduction efficiency of99.999%.

The filtration media used in the above examples can theoretically purify30,000 gallons or more of water if the incoming count of bacteria doesnot exceed about 2,000 ct/cc.

Comparative Example 1

A ceramic filter block having a 0.2 micron rating, in the form of acylindrical block of diameter 2 ½ inches and a length of 20 inches wastested for reduction of E. coli and B. Diminuta by passing water throughthe filtration block at a variable flow rate as indicated below.

Flow at: 15 min. 1.3 gpm 60 min. 0.2 gpm 90 min. 0.1 gpm Total flow 102gallons

The level of E. coli bacteria in the test water was 70000 ct/cc; thetotal dissolved solids of the water was 300 ppm and the hardness of thewater was 200 ppm. The ceramic filter reduced E. coli at an efficiencyof 99.9% at the beginning of the experiment, and at an efficiency of99.99% at the end of the experiment.

The level of B. diminuta in the test water was 60,000 ct/cc, and theflow rate was varied as indicated below:

Flow at: 1 min. 1.6 gpm 2 min. 0.4 gpm

The total dissolved solids content and hardness of the water are asindicated above with respect to testing for E. coli.

The reduction after 90 min. was 92%.

Example 3

A cylindrical block filtration media was prepared by mixing 2 wt % ZnOnanocrystals with 20 wt % high-density polylethylene binder, 7 wt %zirconia and 71 wt % activated carbon, and heating this mixture underpressure to form a block having a diameter of 3 ½ inches and a length of20 inches. The filter was tested for E. coli and B. diminuta removalefficiency as indicated below:

E. coli bacteria reduction: 70,000 ct/cc Flow at: 1 min. 3 gpm 5000gallons: 3 gpm Reduction: 99.999% B. diminuta bacteria reduction: 70,000ct/cc Flow at: 1 min. 3 gpm 3,000 gallons: 3 gpm Reduction at 99.999%3,000 gallons

These examples demonstrate that the use of nanocrystalline titaniumoxide allows the preparation of a polymeric binder-based filtrationmedia having an exact micron rating filter with 99.99% accuracy, andmuch lower than is possible using the polymeric binder without thenanocrystalline particles. Most manufacturers of carbon block filtersadd approximately 5% carbon dust of about 32 microns to improve thefiltration capabilities to obtain a 1 micron rating (i.e., an efficiencyat removing 1 micron particles of 99.99%). In most cases, 5% carbon dustis very difficult to control on the surface of the binder, andsubstantial added amounts of pressure are required to get a uniformproduct. The filter block containing nanocrystals has been found to workwell under compression, from as low as 30 psi to as high as 500 psi. Theperformance of the surface of the nanocrystals is not affected by thepressure, nor by the heat.

What is claimed is:
 1. A filtration media for drinking water,comprising: metal oxide nanocrystals; particles of activated carbon; anda polymeric binder wherein the polymeric binder is present in sufficientamount to adhere the activated carbon particles to each other, andwherein the metal oxide nanocrystals are dispersed in the polymericbinder.
 2. The filtration media of claim 1, wherein the nanocrystals arepresent in an amount ranging from about 0.1 to about 10 wt %, based uponthe total weight of the filtration media.
 3. The filtration media ofclaim 1, wherein the metal oxide is selected from the group consistingof titanium oxide, zinc oxide, and mixtures thereof.
 4. The filtrationmedia of claim 1, further comprising zirconia.
 5. The filtration mediaof claim 1, wherein the nanocrystals have an average particle sizeranging from about 20 nm to about 1000 nm.
 6. The filtration media ofclaim 1, wherein the polymeric binder is selected from the groupconsisting of high-density polyethylene and low-density polyethylene. 7.The filtration media of claim 6, wherein the polymeric binder ishigh-density polyethylene.
 8. The filtration media of claim 1, whereinthe binder adheres the activated carbon particles together in the formof a porous mass or block.
 9. A filtration media in the form of a block,comprising: nanocrystals of zinc oxide or titanium oxide or a mixturethereof in an amount of about 0.5 wt %, based on the total weight of thefiltration media; activated carbon; a polymeric binder.
 10. Thefiltration media of claim 9, wherein the filtration media is capable ofremoving from water bacteria having an average size of about 0.1 micron.11. The filtration media of claim 9, wherein at least a portion of theactivated carbon is present as powder having an average particle size ofapproximately 50-100 microns.
 12. The filtration media of claim 11,wherein the nanocrystals comprise titanium oxide.
 13. The filtrationmedia of claim 12, wherein the polymeric binder comprises about 20%high-density polyethylene.
 14. The filtration media of claim 9, whereinthe nanocrystals, activated carbon, and polymeric binder have beencombined using shear mixing, and wherein the weight ratio ofnanocrystals to polymeric binder was from about 30:1 and about 5:1. 15.The filtration media of claim 14, wherein the weight ratio ofnanocrystals to polymeric binder was from about 20:1 and about 10:1.